1. Field of the Invention
The present invention relates to an ultrasonic transmitter for radiating ultrasonic waves, an ultrasonic transceiver for radiating ultrasonic waves and receiving echoes of the radiated ultrasonic waves, and a sounding apparatus including an ultrasonic transceiver for detecting objects using ultrasonic waves.
2. Description of the Related Art
Today, ultrasonic sounding apparatuses, such as scanning sonars, are widely used for detecting underwater objects (targets). A scanning sonar for detecting underwater objects in all surrounding directions has a generally cylindrical transducer. The scanning sonar forms an ultrasonic transmitting beam oriented in all directions around the transducer by activating vibrating elements arranged on a cylindrical surface of the transducer. Also, the scanning sonar forms a receiving beam oriented in a particular horizontal direction using a specific number of vertically arranged sets, or columns, of vibrating elements centered on that horizontal direction. Typically, this receiving beam is rotated around the transducer to detect underwater objects in a full-circle area by successively switching the columns of vibrating elements.
An ultrasonic transceiver of the aforementioned type of scanning sonar usually includes full-bridge circuits, each including four switching devices, for driving individual vibrating elements. Such an ultrasonic transceiver employs a pulse-duration modulation (PDM) control method which uses a signal having the same frequency as frequency (transmitting frequency) fs of an ultrasonic signal as drive signals for driving the switching devices.
FIG. 8 is an equivalent circuit of a full-bridge circuit used in an ultrasonic transceiver, and FIGS. 9A-9C are diagrams showing a driving pulse signal generated by the full-bridge circuit of FIG. 8 and drive signals supplied to individual switching devices Q1-Q4 of the full-bridge circuit.
Referring to FIGS. 8, 9A-9C, designated by GQ1-GQ4 are gates of the switching devices Q1-Q4, respectively, designated by XD is a vibrating element, designated by C1 and C2 are capacitors for isolating direct-current (dc) components, designated by VB is a driving voltage, and designated by VXD is a load voltage across the vibrating element XD.
As shown in FIG. 8, the full-bridge circuit includes a circuit in which a source of the switching device Q1 is connected to the driving voltage VB, a drain of the switching device Q1 is connected to a source of the switching device Q2, and a drain of the switching device Q2 is grounded and a circuit in which a source of the switching device Q3 is connected to the driving voltage VB, a drain of the switching device Q3 is connected to a source of the switching device Q4, and a drain of the switching device Q4 is grounded. In this full-bridge circuit, the drain of the switching device Q1 is connected to one of terminals of the vibrating element XD via the dc-isolating capacitor C1 and the drain of the switching device Q3 is connected to the other terminal of the vibrating element XD via the dc-isolating capacitor C2.
The drive signals having a frequency fs as shown in FIG. 9B are supplied to the switching devices Q1, Q4 and the drive signals having the same frequency fs as shown in FIG. 9C are supplied to the switching devices Q2, Q3, whereby the aforementioned driving pulse signal having the load voltage VXD as shown in FIG. 9A is produced and fed into the vibrating element XD. The driving pulse signal causes the vibrating element XD to oscillate and radiate the ultrasonic signal into a surrounding environment. The amplitude of oscillation (vibration) of the vibrating element XD can be adjusted by varying an on-duty ratio which is the ratio of the sum of ON periods of the driving pulse signal, or the sum of periods when the load voltage VXD is equal to VB or −VB during a given time duration, to the sum of ON and OFF periods during the same time duration. An example of an ultrasonic transmitter employing the aforementioned type of full-bridge circuit is found in Japanese Patent Application No. 2002-343913, for instance.
An ultrasonic transmitter employing the aforementioned type of full-bridge circuit has a problem in that the number of components increases due to the need for four switching devices in each full-bridge circuit and circuit configuration becomes complicated, resulting in an eventual increase in product cost.
One approach to the resolution of the aforementioned problem is to employ a half-bridge circuit including a pair of switching devices Q1, Q2 as shown in FIG. 10 in an ultrasonic transceiver.
FIG. 10 is an equivalent circuit of the half-bridge circuit, and FIGS. 11A-11C are diagrams showing a driving pulse signal generated by the half-bridge circuit of FIG. 10 and drive signals supplied to the individual switching devices Q1, Q2 of the half-bridge circuit.
Referring to FIGS. 10, 11A-11C, designated by GQ1, GQ2 are gates of the switching devices Q1, Q2, respectively, designated by XD is a vibrating element, designated by C is a capacitor, designated by VB is a driving voltage, and designated by VXD is a load voltage across the vibrating element XD.
As shown in FIG. 10, the half-bridge circuit is a circuit in which a source of the switching device Q1 is connected to the driving voltage VB, a drain of the switching device Q1 is connected to a source of the switching device Q2, a drain of the switching device Q2 is grounded, and the drain of the switching device Q1 is connected to one of terminals of the vibrating element XD via the capacitor C.
The drive signals as shown in FIGS. 11A and 11B are supplied to the switching devices Q1 and Q2, respectively, whereby the aforementioned driving pulse signal having the load voltage VXD as shown in FIG. 11A is produced by the PDM control method and fed into the vibrating element XD. The driving pulse signal causes the vibrating element XD to oscillate and radiate the ultrasonic signal to the exterior. The amplitude of oscillation (vibration) of the vibrating element XD is regulated by varying the on-duty ratio as in the full-bridge circuit.
In the aforementioned circuit configuration in which the vibrating element XD is driven by the half-bridge circuit using the PDM control method, however, there occur harmonics as shown in FIGS. 12A-12C, 13A-13C.
FIG. 12A shows the waveform of a driving pulse signal obtained when an ultrasonic signal is produced at maximum output power by using the half-bridge circuit, FIG. 12B is a frequency spectrum of the driving pulse signal observed at the same time, and FIG. 12C is a frequency spectrum of the ultrasonic signal.
FIG. 13A shows the waveform of a driving pulse signal obtained when an ultrasonic signal is produced at output power reduced to a specific level (−20 dB) by using the half-bridge circuit, FIG. 13B is a frequency spectrum of the driving pulse signal observed at the same time, and FIG. 13C is a frequency spectrum of the ultrasonic signal. Shown in FIGS. 12A-12C, 13A-13C are examples in which the frequency fs of the ultrasonic signal is 81 kHz.
When the vibrating element XD is driven at the maximum output power using the half-bridge circuit, there occur harmonics of which frequencies are odd multiples of the frequency fs (i.e., multiples of the transmitting frequency fs by 3, 5, etc.) as shown in FIGS. 12A-12C. The odd-numbered harmonics, which also occur when the full-bridge circuit is used, can be suppressed by inserting a low-pass filter in an output stage connected to the vibrating element XD.
In a case where the output power is reduced by using the half-bridge circuit, there occur harmonics having all integral multiples of the transmitting frequency fs (i.e., multiples of the frequency fs by 2, 3, 4, etc.) as shown in FIGS. 13A-13C. While almost all of these harmonic components can be suppressed by using a low-pass filter, the second harmonic component can only be removed by use of an additional filter having a high Q factor, rendering circuit design extremely difficult. This is because the second harmonic has a frequency component very close to the transmitting frequency fs of the ultrasonic signal. (In the example of FIG. 12C, the frequency fs of the ultrasonic signal is 81 kHz so that the frequency 2 fs of the second harmonic is 162 kHz.) In addition, the levels of the harmonics vary so little compared to the level of the ultrasonic signal that it is necessary to use a filter having a remarkably large attenuation factor. It is extremely difficult to configure a system which satisfies all these requirements. Even if it is at all possible to design such a system, a series of complicated adjustments would be necessary in manufacturing the system, causing an increase in work load.
One approach to the solution of the above problem would be to employ a pulse-width modulation (PWM) control method in the half-bridge circuit instead of the PDM control method. The term “pulse-width modulation”, or “PWM”, as used in this Specification is a form of pulse-duration modulation, in which the vibrating element XD is driven by a driving pulse signal having a higher frequency fa than the transmitting frequency fs of the ultrasonic signal so that there is output a signal containing multiple pulses having a period Ta within a period Ts of the transmitted ultrasonic signal to pulse-duration-modulate a signal based on the sum of durations of these multiple pulses within the period Ts. When such a driving pulse signal for PDM control operation is supplied to the vibrating element XD, the vibrating element XD resonates at the transmitting frequency fs, and not at the frequency fa of the driving pulse signal, thereby emitting the ultrasonic signal at the transmitting frequency fs. The pulselength of these pulses is determined by comparing a sawtooth-shaped ramp signal having a frequency fc (=fa) higher than the frequency fs of the ultrasonic signal with an analog control signal having the same waveform as the ultrasonic signal, wherein the sawtooth-shaped ramp signal and the analog control signal are synchronized with each other.
When the PDM control method is used with the half-bridge circuit, there occur multiple pulses within the period Ts of the ultrasonic signal even when the output power is reduced. Thus, extremely narrow pulses are not generated within the period Ts so that the half-bridge circuit does not produce such spurious emissions (e.g., the second harmonic) that are difficult to remove.
However, because the frequency fc of the ramp signal and the frequency fa of the driving pulse signal determined by the ramp signal are higher than the frequency fs of the ultrasonic signal, output efficiency decreases as a result of an increase in power loss due to heat generation by the switching devices Q1, Q2 when the half-bridge circuit is operated at the maximum output power.